1. Field of the Invention
The present invention relates to an oxide superconductor containing thallium (Tl), a method for producing the same and an applied apparatus using the oxide superconductor and in particular to a novel oxide superconductor containing thallium and having a high critical temperature.
2. Description of Related Art
Starting from the discovery of La-Ba-Sr-Cu-O, intensive researches and developments have been made on oxide superconductors in application of superconducting technique using liquid nitrogen as a refrigerant based on Y-Ba-Cu-O which has a critical temperature of higher than 90 K (M. K. Wu, J. R. Ashburn, C. J. Torng, Y. Q. Wand and C. W. Chu: Phys. Rev. Lett. 58 (1987), 908).
Remarkable development has been made on materials having high critical temperatures and in 1988, Bi-Sr-Ca-Cu-O oxides were found to be superconductors having a critical temperature in the level of 105K (H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano: Jpn. J. Appl. Phys. 27 (1988), p. L209) and Tl-Ba-Ca-Cu-O oxides were found to be superconductors having a critical temperature in the level of 120 K (Z. Z. Sheng and A. M. Herman: Nature 322 (1988), 55).
These have the structures where copper oxide is inserted in the form of layers between Bi.sub.2 O.sub.2 double layers or Tl.sub.2 O.sub.2 double layers and as the number n of CuO.sub.2 layer, n=1, 2, 3 were confirmed in bulk materials.
Thereafter, Tl-Ba-Ca-Cu-O superconductors in the form of bulk materials were confirmed to have a structure comprising copper oxide is inserted as layers between TlO single layers with the number of CuO.sub.2 layer n=1, 2, 3. (S. S. Parkin, V. Y. Lee, A. I. Nazzal, R. Savoy, R. Beyers and S. J. Laplace: Phys. Rev. Lett. 61 (1988), 750, H. Ihara, M. Hirabayashi, M. Jo, N. Terada, K. Hayashi, A. Negishi, M. Tokumoto, H. Oyanagi, R. Sugie, I. Hayashida, T. Shimomura and S. Ohashi: Proc. 1988, MRS Int. Meet. Advanced Materials, 1988, Materials Research Society).
These are generally produced by milling and mixing powders of carbonates or oxides of strontium and calcium and oxides of bismuth and copper (in case of Tl-Ba-Ca-Cu-O, carbonates or oxides of barium and calcium and oxides of thallium and copper) and firing the mixture at 850.degree.-920.degree. C. for 5 minutes-10 hours in air or oxygen.
However, in a Bi-Sr-Ca-Cu-O material there coexist at least two superconductor phases of critical temperatures in the level of -80 K and in the level of 105 K and unit is difficult to produce each of them as a single phase.
With reference to the Bi-Sr-Ca-Cu-O material, it has been proposed to increase a volume fraction of high temperature phase in the material by replacing a part of Bi with Pb or by producing it under low oxygen partial pressure. However, removal of heterogeneous phase is not sufficient. (M. Takano, J. Takada, K. Oda, H. Kitaguchi, Y. Mimura, Y. Ikeda, Y. Tomii and H. Mazaki: Jpn. J. Appl. Phys. 27 (1988), P. L1041, U. Endo, S. Koyama and T. Kawai: Jpn. J. Appl. Phys. 27 (1988), P. L1476).
Thus, superconductors of high critical current density have not been obtained. On the other hand, in case of Tl-Ba-Ca-Cu-O, since thallium oxide has a high vapor pressure, adjustment of composition is important and synthesis of the single phase is difficult. (See, for example, Nikkei Chodendo (Nikkei: Superconduction), Mar. 21, 1988).
According to the above-mentioned conventional compositions and methods for preparation, critical temperatures of the resulting oxide superconductors are high, namely, 105 K for a Bi type and 118-125 K for a Tl type, but oxide crystals of a single phase cannot be obtained and a plurality of crystal phases are obtained in admixture.
This causes the problems in application of this material to superconducting wire materials and thin film devices that the desired high current density cannot be obtained and the critical temperature is low.
Causes for these problems are still not clear, but it seems that these are caused by the presence of 2-7 kinds of superconductors different in crystal form in the Bi type and Tl-type superconductors, respectively and scattering of composition of raw materials, scattering of oxygen partial pressure and temperature at preparation and change in composition due to change in equilibrium of crystal phase and volatilization of raw materials.